UNIVERSITY  OF  CALIFORNIA   PUBLICATIONS 

* 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY.  CALIFORNIA 


SOME   FACTORS   OF 
DEHYDRATER   EFFICIENCY 


BY 

W.  V.  CRUESS  and  A.  W.  CHRISTIE 


BULLETIN  No.  337 

November,  1921 


UNIVERSITY  OF  CALIFORNIA  PRESS 

BERKELEY 

1921 


David  P.  Barrows,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS  OF  DIVISIONS 

Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

,  Director  of  Resident  Instruction. 

C.  M.  Haring,  Veterinary  Science;  Director  of  Agriculture  Experiment  Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 
Hubert  E.  Van  Norman,  Dairy  Management. 

James  T.  Barrett,  Plant  Pathology;  Acting  Director  of  Citrus  Experiment  Station. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 
John  W.  Gregg,  Landscape  Gardening  and  Floriculture 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Walter  Mulford,  Forestry. 
Fritz  W.  Woll,  Animal  Nutrition. 
W.  P.  Kelley,  Agricultural  Chemistry 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology 
L.  D.  Batchelor,  Orchard  Management. 
J.  C.  Whitten,  Pomology, 
t Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 
R.  L.  Adams,  Farm  Management. 

W.  B.  Herms,  Entomology  and  Parasitology. 

F.  L.  Griffin,  Agricultural  Education. 
John  E.  Dougherty,  Poultry  Husbandry 

D.  R.  Hoagland,  Plant  Nutrition. 

G.  H.  Hart,  Veterinary  Science. 

L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 

DIVISION    OF    VITICULTURE    AND    FRUIT    PRODUCTS 

F.  T.  Bioletti  A.  J.  Winkler 

W  V.  Cruess  G.  Barovetto 

A.  W.  Christie  J.  H.  Irish 
L.  O.  Bonnet 


t  In  cooperation  with  Bureau  of  Public  Roads,  U.  S.  Department  of  Agriculture. 


SOME  FACTORS  OF  DEHYDRATER  EFFICIENCY 

By  W.  V.  CRUESS  and  A.  W.  CHRISTIE 


CONTENTS  page 

Introduction 277 

Plant  Investment 277 

Cost  of  Operation 279 

Fuel  Efficiency 279 

Air  Flow 285 

The  "Parallel  Current"  System 287 

Static  Pressure  and  Recirculation 288 

Air  Distribution 292 

Fans 292 

Trays 292 

Control  of  Humidity 295 

Dipping  Equipment 296 

Summary  and  Conclusions 298 

Introduction. — During  the  past  two  years,  more  than  150  dehydra- 
ters  have  been  built  in  California.  There  are  also  in  existence  not 
less  than  150  driers  of  less  modern  design  built  before  1919.  Some 
of  these  were  erected  merely  as  insurance  against  rain  damage,  but 
many  have  been  used  in  place  of  sun-drying,  as  in  prune  and  apple 
drying.  Many  different  types  are  represented  and  several  different 
systems  of  heat  production  and  heat  conveyance  are  employed.  Obser- 
vations have  been  made  upon  man}^  of  these  plants.  In  several  cases 
direct  comparisons  of  important  types  were  possible.  Because  of 
the  improvements  that  are  rapidty  being  made  in  the  design,  construc- 
tion and  operation  of  dehydraters,  this  publication  must  be  considered 
in  the  nature  of  a  progress  report.  It  is  issued  in  the  hope  that  the 
results,  which  in  many  instances  are  sufficiently  conclusive,  will 
be  of  value  to  operators  and  prospective  purchasers  or  builders  of 
dehydraters.* 

Plant  Investment. — The  cost  of  erecting  dehydraters  varies  greatly 
according  to  the  design,  the  materials  of  construction,  and  the  acces- 
sory equipment. 

During  the  past  two  seasons,  the  cost  of  construction  and  the 
capacity  in  green  tons  per  24  hours  of  several  distinct  types  were 
determined.  Assuming  uniform  rates  of  interest,  depreciation,  insur- 
ance, and  taxes,  and  a  drying  season  of  sixty  days,  calculations  have 


*  To  avoid  confusion  of  terms,  "dehydrater"  is  used  to  designate  the  appa- 
ratus used  for  dehydration,  and  ' '  dehydrato?- ' '  fVi°  aerator  of  this  apparatus. 


278 


UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 


been  made  of  the  proportion  of  the  total  cost  of  dehydration  that 
may  be  assigned  to  the  ' '  fixed  charges ' '  of  plant  investment.  For  com- 
parative purposes,  it  has  been  assumed  that  the  plants  were  operated 
on  grapes  and  prunes  thirty  days  each  and  that  the  drying  ratios  were 
3.5  :1  and  2.5  :1  respectively,  or  an  average  of  3  :1. 

Most  dehydraters  were  actually  operated  during  the  1920  season 
for  less  than  sixty  days.  The  costs  given  in  Table  I  therefore  are, 
on  the  average,  lower  than  the  actual  costs  for  the  past  season.  How- 
ever, by  operating  on  more  kinds  of  fruit,  the  season  can  be  prolonged 
and  the  costs  thereby  lowered. 


TABLE  I 

Cost  of  Dehydration  as  Affected  by  Plant  Investment 


Fix 

ed  Charges  per 

Green  1 

'on 

Capac- 

Total 

Cost 

on  basis  of  60-day  season 

* 

Type  of 

ity 
Green 

First 
Cost 

of 
Plant 

Total 

Plant 

per 

No.f 

Plant 

Tons 

of 

per 

T   , 

De- 

In- 

Dry 

per  24 

Plant 

green 

inter- 
est 

precia- 

sur- 

Taxes 

Total 

Ton 

hours 

ton  24 

tion 

ance 

hours 

A 

Air-blast  tunnel, 
direct  heat 

25 

$12,000 

$480 

$  .56 

$  .80 

$.10 

$  .24 

$1.70 

$5.10 

B 

University  Farm 
type,    Santa 
Clara  County. 

8.6 

5,000 

581 

.67 

.97 

.12 

.26 

1.92 

5.76 

C 

Air-blast  tunnel, 
direct  heat 

24 

14,000 

583' 

.68 

.98 

.12 

.26 

1.94 

5.82 

D 

Air-blast  tunnel, 
direct  heat 

52 

36,000 

596 

.69 

.99 

.12 

.27 

2.07 

6.21 

E 

University  Farm 
type,  Davis 

6 

4,000 

667 

.78 

1.11 

.14 

.29 

2.32 

6.96 

F 

Air-blast,  tunnel 
direct  heat 

35 

25,000 

715 

.83 

1.19 

.15 

.32 

2.49 

7.47 

G 

Air-blast,  tunnel 
direct  heat 

5 

5,500 

1,100 

1.28 

1.83 

.23 

.48 

3.82 

11.46 

H 

Air-blast  cabinet 

12 

15,000 

1,250 

1.46 

2.09 

.26 

.55 

4.36 

13.08 

I 

Small   Oregon 
tunnel  type 

.75 

1,000 

1,330 

1.55 

2.20 

.28 

.59 

4.62 

13.86 

J 

Air-blast  tunnel 

20 

25,000 

1,250 

1.46 

2.08 

.26 

.67 

4.47 

13.41 

K 

Small  stack  type 

.75 

1,500 

2,000 

2.33 

3.55 

.42 

.89 

7.19 

21.57 

L 

Air-blast  stack 
type 

1.50 

4,000 

2,666 

3.11 

4.44 

.55 

1.48 

9.28 

27.84 

M 

Stack  type,  large 
size 

9 

25,000 

2,778 

3.24 

4.63 

.58 

1.23 

9.68 

28.04 

N 

Ceramic  oven 

6 

25,000 

4,167 

4.17 

6.94 

.87 

1.39 

13.37 

40.01 

*Interest  at  7%;  depreciation  at  10%;  insurance  at  2^%  of  }/%  value;  taxes  at  4%  of  2/3  value. 
tThese  letters  are  used  to  designate  the  same  dehydraters  in  subsequent  tables. 


Bulletin  337]       some  factors  of  dehydrater  efficiency  279 

As  stated  above,  the  eosts  reported  in  Table  I  are  based  upon  the 
capacity  for  a  sixty-day  season  and  represent  fixed  charges  for  each 
green  ton  and  each  dry  ton  of  fruit,  respectively,  handled  during  the 
sixty-day  period.  A  depreciation  of  10  per  cent  is  assumed  for  all 
plants,  although  it  varies  greatly  with  the  type  of  construction. 

It  is  evident  from  Table  I  that  most  of  the  dehydraters  listed 
represent  too  large  an  investment  for  the  tonnage  of  fruit  dried.  If 
the  plants  were  operated  throughout  the  year,  the  relative  fixed  charges 
would  be  much  less.  Under  such  conditions,  a  high  initial  investment 
might  be  economically  sound. 

For  the  average  fruit  grower  who  would  operate  during  not  more 
than  two  to  three  months  of  the  year,  the  initial  investment  should  be 
kept  as  low  as  is  compatible  with  efficiency.  This  is  necessary  par- 
ticularly in  cases  where  the  dehydrater  is  to  be  used  for  only  one 
variety  of  fruit.  If  the  plant  is  to  be  used  only  as  insurance  against 
rain  damage,  as  low  cost  of  construction  as  is  compatible  with 
efficiency  is  essential. 

It  has  been  demonstrated  that  a  thoroughly  satisfactory  fruit 
dehydrater  can  be  built  at  a  cost  not  exceeding  (at  present  prices  of 
labor  and  materials)  $500  per  green  ton  capacity  per  24  hours.  This 
includes  cost  of  trays,  cars,  and  all  equipment  used  in  the  dehydrater 
proper,  but  not  dipping  and  packing  equipment.  A  less  expensive, 
but  also  less  efficient  dehydrater  is  often  sufficient  for  insurance 
against  rain  damage. 

Cost  of  Operation. — The  amount  of  labor,  fuel,  power,  and  mater- 
ials used  in  the  operation  of  the  University  Farm  dehydrater  was 
carefully  determined  and  similar  data  were  obtained  for  several  com- 
mercial plants  of  various  designs  and  capacities.  From  these  data, 
the  cost  was  calculated  by  assuming  prices  of  6  cents  per  gallon  for 
fuel  oil,  2!/2  cents  per  kilowatt  hour  for  power,  and  50  cents  per  hour 
for  labor.    Table  II  summarizes  the  results  of  these  calculations. 

These  data  and  calculations,  which  are  supported  by  less  complete 
data  obtained  for  several  plants  not  included  in  Table  II,  lead  to  the 
conclusion  that  the  type  of  dehydrater  consisting  of  a  tunnel  through 
which  the  fruit  is  progressively  moved  on  trucks  and  dried  in  a  blast 
of  recirculated  air  is  the  most  efficient  of  those  now  in  use  in  California. 

Fuel  Efficiency. — We  may  define  fuel  efficiency  as  the  proportion 
of  the  total  heating  value  of  the  fuel  that  is  actually  utilized  in 
evaporating  moisture  from  the  fruit.  For  example,  if  an  amount  of 
fuel  is  burned  sufficient  to  evaporate  1000  pounds  of  water  and  meas- 
urement shows  that  only  500  pounds  of  water  is  evaporated  from  the 

fruit,  the  fuel  efficiency  is  ^-r-  or  50  per  cent. 


280 


UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


TABLE  II 
Comparative  Costs  of  Dehydration* 


Type 

— j 

Fruit 

Cost  per  Green  Ton 

Plant 
No. 

Labor 

Fuel 

Power 
and 
light 

Total 
operat- 
ing 
charges 

Fixed 
charges 

from 
Table  I 

Total 
cost  of 
produc- 
tion 

D 

Direct  Heat 
Air-blast  tunnel 

Prunes 
Prunes 

Grapes 

Prunes 
Prunes 
Grapes 
Prunes 

$3.19 
4.75 

4.16 

4.80 
5.70 
4.56 
9.75 

$  .94 
1.28 

2.05 

1.63 
1.70 
1.44 
3.26 

$  .48 
.56 

.45 

.55 

1.18 

.59 

.20 

$4.61 
6.59 

6.66 

6.98 

8.58 

6.59 

13.21 

$2.07 
1.92 

2.32 

4.36 

4.47 

13.37 

9.68 

$6.68 

B 

E 

H 
J 

University  Farm  type 
Santa  Clara  Co. 

University  Farm  type 
Davis 

Air-blast  Cabinet 

Air-blast  tunnel 

8.51 

8.98 

11.34 
13.05 

N 

Ceramic  Oven 

19.96 

M 

Stack  -type    Gravity 
Air  Flow 

22.89 

Approximately  1000  B.  T.  U.  (British  Thermal  Units)  are  required 
to  evaporate  one  pound  of  water.  Fuel  oil  furnishes  on  complete 
combustion  approximately  135,000  B.  T.  U.  per  gallon.  Therefore 
one  gallon  of  oil  will  evaporate  at  100  per  cent  efficiency  about  135 
pounds  of  water.  Since  the  amount  of  water  evaporated  in  a  given 
dehydrater  is  obtained  by  subtracting  the  weight  of  dry  fruit  from 
that  of  green  fruit,  we  have  by  application  of  the  above  facts  the 
following  simple  formula  for  the  calculation  of  the  approximate  fuel 
efficiency : 

Pounds  green  fruit  —  Pounds  dry  fruit        ^     ,    ffl  . 
Gallons  of  oil  consumed  X  135 

The  exactness  of  this  formula  will  vary  slightly  with  the  tempera- 
ture of  the  air  used  in  drying  and  with  the  heat  value  of  the  oil.  It 
is  sufficient  for  comparative  purposes. 

Application  of  this  formula  to  data  secured  at  several  dehydraters 
during  the  past  season  gives  the  results  presented  in  Table  III. 

The  air-blast  tunnel  type  of  dehydrater  is  shown  by  the  data  in 
Table  III  to  be  much  more  efficient  than  the  gravity  air-flow  type. 
These  results  are  confirmed  by  similar  observations  made  in  1919  and 
by  data  of  the  1920  season  not  reported  in  Table  III. 


♦Figures  for  labor,  fuel,  power,  and  light  represent  actual  observations,  while  fixed  charges  are 
based  upon  cost  of  plant  and  an  operating  season  of  sixty  days. 


BULLETIN   337]         SOME  FACTORS   OF  DEHYDRATER  EFFICIENCY 


281 


14 Direct  heat"  dehydraters  in  all  instances  gave  high  fuel  efficiency. 
This  was  to  be  expected  because  in  this  type  of  dehydrater,  stack  losses 
are  eliminated. 


Fig.  1. — Students  of  fruit  products  operating  University  Farm  dehydrater. 

In  general,  it  was  possible  to  obtain  higher  fuel  efficiency  in  the 
dehydration  of  free  drying  fruits,  such  as  apples  and  apricots,  than 
of  fruits  which  case-harden,  such  as  pears  and  prunes.  By  "case- 
hardening"  is  meant  the  dessication  or  over-drying  of  the  surface  of 
fruits,  which  makes  them  almost  impervious  to  the  escape  of  moisture 
from  within  and  thereby  greatly  lengthens  the  drying  time.  If  the 
humidity  and  the  recirculation  of  the  air  were  under  perfect  control, 
differences  in  fuel  efficiency  for  different  fruits  would  be  lessened. 


282 


UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


TABLE  III 

Comparative  Fuel  Efficiencies  of  Several  Types  of 

Dehydraters 


Plant 

No. 


Type  of  Dehydrater 


Fruit  Dried 


Fuel  Efficiency 
per  cent 


E 

N 

F 

N 

E 

E 

B 

J 

J 

E 
H 
M 
M 


University  Farm  type,  Davis 

Ceramic  Oven 

Air-blast  Tunnel  Direct  Heat 

Ceramic  Oven 

University  Farm  type,  Davis 

University  Farm  type,  Davis 

University  Farm  type,  Santa  Clara  Co. . . . 

Air-blast  Tunnel 

Air-blast  Tunnel   (same  design  as  pre 

ceding  but  in  another  location) 

University  Farm  type,  Davis 

Air-blast  Cabinet 

Stack  type  Gravity  Air  Flow 

Small  Stack  type  Gravity  Air  Flow 


Apricots 

Apples 

Grapes 

Grapes 

Peaches 

Pears 

Prunes 

Prunes 

Prunes 
Grapes 
Prunes 
Prunes 
Apricots 


58 
50 
48 
44 
43 
43 
42 
39 

38 
38 
30 
24 
14 


A  well-designed  dehydrater  should  have  a  fuel  efficiency  of  at 
least  40  per  cent  in  drying  prunes  or  grapes.  An  efficiency  above 
50  per  cent  is  very  difficult  to  attain  under  usual  conditions.  With 
freely  drying  fruit  during  hot  weather,  50  per  cent  efficiency  may 
be  exceeded,  as  in  the  first  test,  Table  III.  Where  the  products  of 
combustion  are  passed  directly  over  the  fruit,  it  has  been  found  that 
over  90  per  cent  of  the  heat  generated  is  taken  up  by  the  air.  Where 
a  furnace  and  an  efficient  system  of  radiating  flues  are  used,  it  is 
possible  to  transmit  as  much,  as  85  per  cent  of  the  heat  generated  in 
the  furnace  to  the  air  used  in  drying.  This  was  demonstrated  at  the 
University  Farm  dehydrater  during  1919  and  1920.  #The  "direct 
heat"  system  requires  the  use  of  fuel  which  burns  free  of  soot,  smoke, 
or  objectionable  odor.  For  this  reason,  more  expensive  fuel  is  neces- 
sary than  for  the  radiation  system.  This  fact  tends  to  counterbalance 
the  greater  efficiency  of  the  direct  heat  system.  There  has  been  no 
opportunity  to  compare  the  fuel  efficiencies  of  steam-heated  dehydra- 
ters with  those  of  dehydraters  heated  by  furnaces  or  direct  heat.  It 
is  known,  however,  that  only  55  to  65  per  cent  of  the  heat  value  of  fuel 
is  transmitted  to  steam,  though  as  much  as  95  per  cent  of  this  55  to  65 
per  cent  may  be  transferred  to  the  air  used  in  drying. 

Crude  oil  and  stove  oil  are  the  fuels  in  most  common  use  in 
dehydraters  in  California.  The  latter  burns  practically  without  soot, 
if  a  forced  draft  furnace  is  used.    Crude  oil  even  in  the  best  burners 


BULLETIN   337J  SOME  FACTORS   OF   DEHYDRATER  EFFICIENCY 


283 


to 


< 

o 

GO 
r+-' 

P 
H 


-s 
P 


u 

p 
< 


to 

O 
p 
a 


- 


o 


CO 

CO 


x 

P 

Q 

p 
o 

P 
r+ 
O 

d 

5. 

o 

'-i 

05 


P 


o 

p 


284 


UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


forms  an  appreciable  amount  of  soot.  This  makes  it  necessary  to  seal 
all  flue  connections  tight,  so  that  no  soot  is  carried  through  the  drying 
tunnel  to  contaminate  the  fruit.  Observation  has  demonstrated  that 
this  is  a  real  difficulty. 

Functions  of  the  Air. — Air  serves  two  purposes  in  the  dehydration 
of  fruits.  It  conveys  to  the  fruit  the  heat  necessary  to  evaporate  the 
surplus  moisture  and  it  carries  away  the  water  vapor  after  evapora- 
tion has  taken  place.  Much  more  air  is  required  for  the  former  than 
for  the  latter  function. 


TABLE  IV 

Examples  of  Air-Flow  Measurements  and  Drying  Times 


Plant 


F 
C 
A 
O 

E 
G 
B 
M 

M 


N 
N 


Type  of 
Dehydrater 


Air-blast  direct  heat. 
Air-blast  direct  heat. 
Air-blast  direct  heat. 
Air-blast  tunnel  batch 

type 

University  Farm  type 
Air-blast  direct  heat 
University  Farm  type 
Stack    type    Gravity 

Air  Flow 

Stack    type    Gravity 

Air  Flow 

Ceramic  Oven 

Ceramic  Oven 


Fruit 

Velocity 
of  air 
across 
trays. 
Linear 

feet  per 
minute 

Total 
volume 

of  air. 

Cubic 
feet  per 
minute 

Volume 
of  air  per 
100  sq.  ft. 

of  tray 

surface 
Cu.  i\.  per 

minute 

Grapes 

424 

20,800 

290 

Grapes 

485 

15,800 

275 

Prunes 

510 

44,000 

255 

Grapes 

450 

17,500 

250 

Grapes 

265 

8,600 

250 

Grapes 

197 

7,486 

235 

Prunes 

357 
Less  than 

11,390 

200 

Prunes 

20 
Less  than 

4,500 

130 

Prunes 

20 
Less  than 

4,800 

100 

Grapes 

20 

6,017 

110 

Apples 

20 

6,660 

105 

Approx- 
imate 
drying 
time. 
Hours 


24 
24 
24 

18-24 
18-30 

22 

30 

30 
36 

60 

18 


To  evaporate  one  pound  of  water  at  the  average  temperature  used 
in  dehydrating  fruit  requires  the  heat  furnished  by  1750  cubic  feet 
of  air  dropping  40°  F.,  but  one  pound  of  water  vapor  will  saturate 
only  235  cubic  feet  of  air  at  110°  F.  For  example,  if  dry  air  enters 
the  drying  chamber  at  150°  F.  and  leaves  it  saturated  with  water  at 
110°  F.,  235  cubic  feet  will  carry  away  one  pound  of  water.  But 
under  these  conditions  with  a  40°  temperature  drop,  1750  cubic  feet 
is  necessary  to  evaporate  one  pound  of  water,  or  more  than  7  times 
as  much  as  that  required  to.  carry  away  the  evaporated  moisture.  The 
ratio  will  be  less  than  7  :1  unless  the  entering  air  is  perfectly  dry, 
or  the  escaping  air  completely  saturated  with  moisture.    For  example, 


Bulletin  337]         SOME  FACTORS  OF  DEHYDRATER  EFFICIENCY  285 

if  the  entering  air  is  10  per  cent  saturated  with  moisture  vapor  at 
150°  F.,  and  the  exhaust  air  is  at  110°  F.  and  saturated  with  moisture 
vapor.  335  instead  of  235  cubic  feet  will  be  required  to  absorb  one 
pound  of  water  vapor  from  the  fruit.  Again,  if  the  entering  air  at 
150°  F.  is  10  per  cent  saturated  and  the  exhaust  air  at  110  F.  is  only 
75  per  cent  saturated  with  moisture,  522  cubic  feet  of  air  will  be 
required  to  remove  one  pound  of  water  vapor  from  the  fruit ;  the  ratio 
between  the  amount  of  air  required  to  furnish  heat  and  that  necessary 
to  remove  the  water  vapor  becoming  3.35  :1.  For  simplicity  the  above 
calculations  disregard  the  slight  differences  in  the  volume  of  air  caused 
by  changes  in  temperature. 

Knowing  the  tons  of  green  fruit  per  charge,  the  drying  ratio  of 
the  fruit  and  the  estimated  drying  time,  it  is  possible  to  calculate  the 
minimum  air-flow  requirement  for  a  dehydrater.  For  example,  if  a 
dehydrater  is  to  hold  ten  tons  of  prunes  with  a  drying  ratio  of  2y2  :1, 
and  is  to  dry  the  fruit  in  24  hours,  it  will  be  necessary  to  remove 
12,000  pounds  of  water  per  24  hours  or  8.3  pounds  per  minute.  If 
the  temperature  drop  is  40°  F.,  there  will  be  required  8.3  X  1750, 
or  14,525  cubic  feet  of  air  per  minute.  If  various  heat  losses  are 
included,  the  designer  should  allow  at  least  20,000  cubic  feet  of  air  per 
minute  for  a  dehydrater  of  this  size. 

The  importance  of  adequate  air  flow  cannot  be  over-emphasized. 
More  dehydraters  have  failed  because  of  insufficient  air  supply  than 
for  all  other  reasons  combined.  It  must  be  realized  that  at  least  five 
times  as  much  air  is  usually  necessary  to  furnish  heat  for  drying  as 
is  required  for  the  removal  of  the  evaporated  moisture. 

With  these  principles  in  mind,  the  air  flow  in  a  number  of 
dehydraters  was  studied  during  the  past  two  seasons.  The  results 
obtained  are  given  in  Table  4.  The  air  flow  in  all  instances  was 
determined  by  means  of  an  anemometer.  This  instrument  was  placed 
in  various  positions  in  the  drying  tunnels  or  air  ducts  and  the  results 
represent  in  each  case  the  average  of  several  determinations.  The 
anemometer  is  in  common  use  by  heating  and  ventilating  engineers. 
It  is  an  inexpensive  and  useful  instrument  for  the  operator  of  a 
dehydrater. 

Some  typical  air-flow  measurements  are  given  in  Table  IV. 
Unfortunately,  an  exact  comparison  of  these  data  on  air-flow  and 
drying  times  is  impossible  because  of  the  effect  of  many  other  variables, 
such  as  variety  of  fruit,  treatment  before  drying,  load  per  square  foot, 
temperature,  humidity,  moisture  content  of  finished  product,  etc.,  each 
of  which  directly  affects  the  drying  time.  Experiments  are  now 
in  progress  in  which  it  is  hoped  to  establish  more  exactly  the  inter- 


286 


UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


relations  of  all  the  factors.  However,  the  writers  feel  from  their 
observations  of  the  past  two  seasons  that  air-blast  dehydraters,  to  be 
reasonably  efficient,  should  be  supplied  with  a  velocity  of  air  over  the 
trays  of  not  less  than  500  feet  per  minute  and  a  volume  of  at  least 
250  cubic  feet  per  minute  for  each  100  square  feet  of  tray  surface. 
Most  of  the  dehydraters  tested  were  operated  at  temperatures  of 
165°  F.  to  185°  F.  at  the  "finishing  point."     The  best  quality  with 


Fig.  3. 


-Anemometer  used  to  determine  the  velocity  of  air  in  dehydraters. 


most  fruits  is  most  readily  attained  by  finishing  at  temperatures  not 
in  excess  of  150°  F.,  but  with  the  same  volume  of  air  per  unit  of 
drying  surface  drying  is  less  rapid  than  at  165°  F.  Within  certain 
limits,  the  same  rate  of  drjdng  at  lower  temperatures  can  be  main- 
tained by  increasing  the  volume  of  air.  Owing  to  the  facts  that 
horsepower  increases  very  rapidly  with  increase  in  air  velocity  and 
that  a  certain  minimum  time  is  required  for  any  fruit  to  give  up 
its  moisture  to  the  surrounding  air,  increase  of  air  velocit}^  beyond 
a  certain  maximum  becomes  uneconomical.  Although  this  maximum 
will  vary  greatly  with  the  variety  of  fruit  and  its  preliminary  treat- 
ment, the  maximum  efficient  velocity  for  most  products  would  prob- 
ably not  exceed  1100  feet  per  minute.  The  horse  power  necessary 
to  operate  a  fan  increases  proportionately  to  the  cube  of  the  revolutions 


BULLETIN   337]         SOME  FACTORS   OF  DEHYDRATER  EFFICIENCY  287 

per  minute,  and  roughly  in  proportion  to  the  volume  of  air  delivered 
and  to  the  velocity  across  the  trays.  Taking  all  factors  into  con- 
sideration, economical  drying  can  best  be  obtained  by  a  velocity  of 
not  less  than  500  feet  per  minute  for  fruits  which  case-harden,  and 
a  velocity  of  at  least  750  feet  per  minute  for  freely  drying  fruits.  In 
the  drying  of  grapes,  sliced  apples,  and  other  freely  drying  sub- 
stances, it  is  probable  that  velocities  of  800  to  1000  feet  per  minute 
would  sufficiently  accelerate  drying  to  compensate  for  the  increased 
cost  of  power  for  the  fan.  Dehydraters  depending  to  a  considerable 
extent  upon  direct  radiation  of  heat  require  less  air  than  air-blast 
dehydraters  to  accomplish  the  same  amount  of  drying  within  the  same 
time.  Such  dehydraters  are,  however,  limited  in  their  rate  of  drying 
by  the  amount  of  heat  reaching  the  fruit  by  direct  radiation  and  by 
the  velocity  of  air  flow  which  it  is  possible  to  obtain  by  natural  draft. 
Attempts  to  equip  stack  driers  with  fans  have  not  proved  satisfactory 
because  their  construction  does  not  permit  uniform  air  distribution. 
Our  observations  on  the  air  flow  in  natural  draft  dehydraters  are 
insufficient  to  base  recommendation  on  regarding  minimum  air-flow 
requirements.  However,  natural-draft  dehydraters  even  of  the  most 
approved  design  give  a  slower  rate  of  drying,  a  less  uniformly  dried 
product,  and  a  lower  fuel  efficiency  than  the  best  air-blast  dehydraters. 
These  advantages  of  the  latter  are  obtained  for  a  smaller  initial  plant 
investment,  when  the  drj-ing  capacity  is  considered. 

The  "Parallel  Current"  System  of  Dehydration. — In  most  tunnel 
dehydraters  the  fresh  fruit  enters  at  the  air  exhaust  end  and  the 
dried  fruit  leaves  at  the  air  intake  end  of  the  drying  compartment. 
During  drying,  the  fruit  is  moved  from  air  of  moderate  temperature 
(100°  F.  to  120°  F.)  at  the  start  of  drying  to  temperatures  of  150°  F. 
to  190°  F.  near  the  end  of  the  drying  period.  This  is  termed  the 
"counter  current"  system.  During  the  first  stages,  very  little  drying 
occurs  because  of  the  moist  condition  and  relatively  low  temperature 
of  the  air.  The  drying  process  is  completed  in  air  of  high  temperature 
and  low  relative  humidity,  conditions  that  favor  case-hardening  and 
scorching  of  the  fruit. 

In  the  so-called  "parallel  current  system,"  the  fruit  enters  at  the 
air  intake  end  of  the  drying  compartment  and  is  taken  from  the 
dehydrater  at  the  air  exhaust  end.  In  other  words,  the  drying  process 
is  started  in  hot,  dry  air  and  is  completed  in  warm,  moist  air.  For 
some  fruits  this  system  possesses  the  following  advantages : 

1.  Evaporation  of  the  surplus  moisture  is  very  rapid  during  the 
initial  stages  of  the  drying  period  when  the  fruit  is  moist  and  in  the 
best  condition  to  give  up  its  water. 


288  UNIVERSITY  OP  CALIFORNIA EXPERIMENT  STATION 

2.  The  wet  fruit  is  more  nearly  at  the  temperature  of  the  wet- 
bulb  thermometer  because  the  fruit  contains  sufficient  moisture  to 
maintain  a  rapid  rate  of  evaporation  which  reduces  its  temperature 
proportionately.  This  permits  higher  drying  temperatures  than  are 
now  used,  thus  still  further  increasing  the  rate  of  drying.  In  the 
' '  counter  current ' '  system  the  fruit  near  the  end  of  the  drying  process, 
because  of  its  low  moisture  content  and  slow  rate  of  drying,  is  very 
apt  to  approach  the  temperature  of  the  hot,  dry  air  and  become 
scorched  and  carmelized.  The  "parallel  current"  system  takes  fuller 
advantage  of  the  great  drying  power  of  air  direct  from  the  heating 
chamber. 

3.  The  fruit  gradually  progresses  during  drying  toward  a  region 
of  lower  temperature  and  higher  humidity  so  that  scorching  and  over- 
drying  are  avoided. 

4.  The  fruit  emerges  after  drying  at  a  relatively  low  temperature 
so  that  much  less  heat  is  carried  to  the  outside  atmosphere  by  heated 
cars,  trays,  and  fruit  than  is  the  case  with  the  "counter  current" 
system.     The  "parallel  current"  system  therefore  conserves  heat, 

A  preliminary  test  of  this  method  was  made  in  a  large  commercial 
dehydrater.  Two  carloads  of  grapes  which  received  the  high  initial 
temperature,  dried  so  rapidly  that  it  was  necessary  to  remove  them 
from  the  tunnel  several  hours  before  cars  which  had  received  a  low 
initial  temperature.  Further  tests  on  the  "parallel  current"  system, 
as  applied  to  apples  and  cherries  have  been  conducted  in  the  Fruit 
Products  Laboratory  and  in  commercial  plants  with  favorable  results. 

Static  Pressure  and  Recirculation. — Recirculation  of  a  large  pro- 
portion of  the  air  used  in  drying  conserves  fuel  and  makes  possible 
the  regulation  of  the  humidity  of  the  air,  which  is  a  factor  of  great 
importance  in  the  drying  of  fruits  which  tend  to  case-harden.  Under 
average  conditions,  from  five  to  eight  times  as  much  air  is  required 
for  heat  transfer  as  for  moisture  removal.  Therefore,  it  is  often 
possible  to  return  75  per  cent  or  more  of  the  air  to  the  heating 
chamber.  This  fact  has  been  successfully  utilized  in  many  of  the 
dehydraters  built  during  1919  and  1920. 

Return  of  the  air  to  the  heating  chamber  doubles  the  distance 
traveled  and  consequently  increases  the  load  upon  the  fan  and  motor. 
The  return  air  duct,  therefore,  must  be  of  large  cross-section  in  order 
that  the  static  pressure  (air  friction)  therein  shall  not  be  excessive. 
If  this  return  duct  is  too  small,  it  will  greatly  reduce  the  volume  of 
air  handled  by  the  fan  and  tend  to  counterbalance  the  advantages  of 
recirculation. 


Bulletin  337]        some  factors  OF  dehydrater  efficiency 


289 


For  the  same  reason,  the  air  delivery  duct  between  the  fan  and 
the  drying  tunnel  must  be  as  short  and  direct  as  possible  and  large 
enough  to  avoid  serious  static  pressure.  The  velocity  of  the  air  in 
ducts  leading  to  and  from  the  fan  should  not  exceed  the  velocity  of 
the  air  across  the  trays,  i.e.  500  to  1100  feet  per  minute.  Velocities 
of  over  3500  feet  per  minute  have  been  observed  in  the  small  air 
passages  of  several  dehydraters.  This  causes  very  high  static  pressure 
and  low  air  flow  in  the  drying  compartment. 


Fig.  4. — Recording  thermometer,  showing  a  24-hour  temperature  record  during 
dehydration  of  grapes  in  the  University  Farm  dehydrater. 

Static  pressure  may  be  considered  as  the  pressure  necessary  to 
overcome  the  frictional  resistance  offered  to  the  flow  of  air.  It  is 
measured  by  a  Pitot  tube  and  is  expressed  as  "inches  of  water  pres- 
sure.' There  are  three  pressures  to  consider  in  Pitot  tube  measure- 
ments :  first,  static  or  frictional  resistance  pressure ;  second,  velocity 
pressure  due  to  the  velocity  of  the  air ;  and  third,  the  total  or  impact 
pressure,  which  is  the  sum  of  the  static  and  velocity  pressures.  Since 
velocity  pressure  is  the  difference  between  the  total  pressure  and  static 
pressure,  any  increase  in  static  pressure  will  result  in  a  decrease  in 


290 


UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


velocity  pressure  and  consequently  a  decrease  in  volume  of  air  de- 
livered. The  effect  of  increased  static  pressure  can  be  overcome  to 
a  certain  extent  by  increasing  the  speed  of  the  fan,  but  this  in  turn 
involves  increased  expenditure  of  power. 


TABLE  V 

Comparative  Static  Pressures  in  Various  Dehydraters 


Type  of  Dehydrater 

Static  Pressure  in  Inches  of  Water 

Plant 

No. 

At 

Fan  Intake 

(Suction) 

At 

Fan  Discharge 

(Pressure) 

Total 

E 

University   Farm,    Davis,   with 
partial  recirculation 

-  .65 

-  .93 

-  .81 

-1.63 
-1.76 

-  .85 

-  .16 

.51 

.86 

.43 

.07 
.13 

.67 
.74 

.52 

E 

N 

University   Farm,    Davis,   with 

total  recirculation 

Ceramic  Oven 

.56 
1.16 

G 
G 

Air-blast  tunnel,  direct  heat 
Air-blast  tunnel,  direct  heat,  no 
recirculation 

1.79 
1.24 

G 

Air-blast    tunnel,    direct    heat, 
comolete  recirculation 

2  63 

0 

Air-suction  tunnel,  no  recircula- 
tion  

1.70 

0 

Air-suction   tunnel,    partial   re- 
circulation  

1.89 

0 

Air-suction  tunnel,   total  recir- 
culation  

2.19 

P 

Air-suction    tunnel,    tray    and 
slide  type 

.70 

B 

University  Farm  type 

No  recirculation 

1.52 

Total  recirculation 

.90 

P 

No    recirculation.      Smaller 
model    than  above   dehy- 
drater P 

1.28 

The  excessive  static  pressures  found  to  exist  in  several  dehydraters 
ar^  due  to  the  use  of  very  narrow  and  crooked  air  ducts.  Air  ducts 
should  contain  as  few  bends  as  possible ;  where  bends  are  necessary, 
these  should  be  wide,  to  reduce  air  friction  to  a  minimum. 

Taking  into  account  the  precautions  necessary  in  such  measure- 
ments, determinations  were  made  of  the  static  pressure  in  several 
dehydraters,  with  the  results  shown  in  Table  V.  It  is  evident  from 
this  table  that  the  assumption  frequently  made  that  the  static  pressure 
in  tunnel  dehydraters  would  be  y2  to  1  inch  is  too  low.  Estimates 
based  on  this  assumption  have  led  to  disappointment  in  the  perform- 
ance of  certain  dehydraters.     Since  the  air  velocities  in  the  different 


BULLETIN   337]  SOME  FACTORS   OF   DEHYDRATER   EFFICIENCY 


291 


plants  tested  were  not  the  same,  no  exact  comparisons  of  static  pressures 
can  be  made,  because  static  pressure  varies  directly  as  the  square  of 
the  air  velocity.  The  effect  of  high  air  velocity  on  static  pressure 
is  shown  in  Plant  0,  in  which  high  air  velocity  is  used.  The  volume 
of  air  delivered  by  a  fan  decreases  as  static  pressure  increases.  Thus, 
a  Number  7  Sirocco  fan  revolving  at  350  r.p.m.  has  a  rated  capacity 
of  33,000  cubic  feet  per  minute  at  one  inch  static  pressure,  while  at 
the  same  speed  and  two  inches  static  pressure  the  capacity  is  reduced 


Fig.  5. — Two  types  of  speed  indicators  for  determining  the  speeds  of  motors 
and  fans. 

to  17,000  cubic  feet.  A  direct  test  was  made  at  the  University  Farm 
dehydrater  to  determine  the  effect  of  the  size  of  the  return  flue  on 
the  volume  of  air  delivered.  With  a  return  flue  of  two  square  feet 
in  area  and  complete  recirculation,  5150  cubic  feet  of  air  per  minute 
passed  through  the  return  flue.  When  the  return  flue  was  increased 
to  four  square  feet  in  area,  the  volume  of  air  increased  to  6250  cubic 
feet  per  minute  or  an  increase  of  over  20  per  cent.  In  one  type  of 
dehydrater  manufactured  in  California  the  cross-section  area  of  the  air 
duct  connecting  the  fan  and  the  drying  tunnel  was  about  1.7  per  cent 
of  the  cross-section  area  of  the  drying  compartment.  Static  pressure 
in  this  case  so  reduced  air  flow  that  the  drying  time  of  one  variety 
of  fruit  was  about  four  times  as  long  as  in  well  designed  plants. 


292  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

Air  Distribution. — Some  dehydraters,  though  equipped  with  fans 
and  motors  of  sufficient  capacity,  have  not  dried  the  fruit  so  rapidly 
as  they  should  have  done  with  the  volume  of  air  furnished.  Investi- 
gation proved  that  an  excessive  proportion  of  the  air  was  flowing  above 
or  below  the  cars,  between  the  cars,  or  between  the  cars  and  the  walls. 
Such  air,  of  course,  accomplishes  no  drying. 

A  typical  instance  will  indicate  the  importance  of  this  factor.  A 
certain  dehydrater  loaded  with  about  25  tons  of  prunes  gave  air-flow 
readings  behind  the  last  car  as  shown  in  Table  VI,  Column  2.  Baffles 
were  then  placed  below  the  car  frames  and  on  the  ceiling  of  the 
dehydrater,  and  the  air-flow  readings  given  in  Column  3  were  then 
obtained.  Before  baffles  were  installed,  over  75  per  cent  of  the  total 
volume  of  air  passing  through  the  dehydrater  was  flowing  beneath  or 
above  the  cars  and  accomplishing  little  or  no  drying.  The  installation 
of  baffles  increased  the  velocity  of  air  flow  across  the  trays  about 
42  per  cent  and  shortened  the  drying  time  materially.  In  other  dehy- 
draters the  velocity  of  the  air  was  much  higher  near  the  top  of  the  cars 
of  trays  than  in  the  center  of  the  load  or  near  the  bottom.  In  a  few 
instances  the  greatest  velocity  was  found  near  the  bottom  of  the  stack 
of  trays.  Such  irregularity  of  air  flow  causes  uneven  rates  of  drying. 
Uneven  distribution  can  be  overcome  by  means  of  suitable  dampers  at 
one  or  both  ends  of  the  tunnel.  Also  if  the  intake  or  discharge  of  the 
fan  be  placed  below  the  center  of  the  tunnel,  it  will  counterbalance 
the  tendency  of  heated  air  to  rise.  By  the  use  of  an  anemometer  the 
velocity  of  the  air  in  different  parts  of  the  dehydrater  can  be  deter- 
mined and  it  is  then  usually  possible  to  correct  faulty  air  distribution. 

Fans. — Previous  conclusions  given  in  our  Bullettin  322,  The 
Evaporation  of  Grapes,  in  regard  to  the  relative  efficiency  of  different 
types  of  fans  were  confirmed  by  observations  during  the  past  season. 
Disc  fans  are  not  satisfactory  for  use  in  long  tunnels  or  against  high 
static  pressure  because  they  cannot  force  air  against  high  resistance. 
They  can  be  used  for  short  tunnels  in  which  there  is  wide  clearance 
(at  least  3  inches)  between  trays.  Such  fans  are  inexpensive  and 
therefore  suitable  for  "rain  damage  insurance"  driers.  However,  for 
the  greatest  efficiency  a  multivane  or  steel  plate  fan  is  necessary, 
despite  the  greater  cost.  These  fans  are  capable  of  overcoming  high 
static  pressure  and  make  possible  the  use  of  a  much  higher  air  velocity 
than  can  be  obtained  with  the  disc  fan. 

Trays. — The  size  and  design  of  trays  in  use  vary  almost  as  much 
as  the  dehydraters  themselves.  Although  direct  comparisons  were 
not  made  of  all  styles  of  trays,  the  relative  efficiency  of  the  more 
common  types  were  determined. 


Bulletin  337]       some  factors  of  dehydrater  efficiency 


293 


TABLE  VI 

Effect  on  Air  Distribution  of  Proper  Placing  of  Baffles 


Location  of  Test 


Velocity  of  air  between  trays  near  top  of  car 

Velocity  of  air  between  trays  near  bottom  of  car 

Velocity  of  air  below  cars 

Velocity  of  air  above  top  tray 


Velocity 

Before 

Installing 

Baffles 

Ft.  per  Min. 


320 
400 

1500 
2800 


Velocity 

After 

Installing 

Baffles 

Ft.  per  Min. 


600 
420 

500 
500 


It  was  found  that  trays  3'  X  6'  or  3'  X  8'  were  not  satisfactory 
where  the  air  flows  across  the  greater  length  of  the  tray,  because  the 
fruit  on  the  end  of  the  tray  nearest  the  heating  chamber  dried  much 
more  rapidly  than  that  on  the  opposite  end  of  the  tray.  Air  during 
its  passage  across  such  long  trays  loses  a  great  deal  of  its  drying  power 
because  of  increased  humidity  and  decreased  temperature.  Reversal 
of  the  air  current  during  drying  does  not  entirely  overcome  the  defect 
because  there  is  then  a  tendency  for  the  fruit  in  the  center  of  the 
tray  to  dry  more  slowly  than  that  at  the  ends.  It  is  believed  that 
forty  inches  should  be  the  maximum  length  for  trays,  unless  the 
3'  X  6'  or  3'  X  8'  trays  are  placed  crosswise  to  the  direction  of  the 
air  flow. 

Very  large  trays  are  necessarily  ' '  two  man ' '  size  and  inconvenient 
for  stacking.  Trays  36"  X  36"  or  36"  X  40"  have  proved  very  satis- 
factory for  tunnel  dehydraters  and  are  more  durable  than  larger 
trays.  Where  it  becomes  necessary  to  use  field  trays,  as  in  "rain 
damage''  dehydraters,  blocks  must  be  placed  between  them  so  that 
there  is  at  least  two  inches  of  free  space  between  their  edges.  Where 
this  is  not  done,  air  flow  is  so  much  impeded  that  drying  becomes 
excessively  slow.  One-inch  blocks  are  usually  sufficient  for  2'X3r 
raisin  trays  because  the  sides  of  these  trays  are  only  about  %  inch 
in  height  and  the  ends  1%  inches. 

A  small  stack  evaporator  was  used  experimentally  at  the  Univer- 
sity Farm  during  1920.  The  fruit  was  dried  on  2'  X  3'  wooden  trays 
with  3-inch  sides  and  slat  bottoms.  The  trays  were  satisfactory  for 
this  type. 

Slat  bottom  trays  are  preferrable  to  galvanized  iron  screen  trays 
for  all  fruits  that  require  sulfuring,  such  as  white  grapes,  apricots, 
peaches,  pears,  and  apples.  Galvanized  screen  trays  corrode  rapidly 
in  sulfur  fumes  and  impart  a  disagreeable  metallic  flavor  to  the  fruit. 
Peeled  peaches  and  very  juicy  fruits  stick  badly  to  wooden  trays, 


294 


UNIVERSITY  OP  CALIFORNIA EXPERIMENT  STATION 


although  sticking  is  slightly  reduced  by  coating  the  trays  with  a 
neutral  mineral  oil  each  time  they  are  used.  Many  different  kinds 
of  paints  and  protective  coatings  for  screen  trays  were  tested.  So  far, 
no  satisfactory  material  has  been  discovered.  All  the  paints  tested 
either  soften  and  stick  to  the  fruit  because  of  the  high  temperature 
or  become  brittle  and  scale  off  the  screen  during  the  removal  of  the 
dried  fruit. 


Fig.  6. — Angle  stem  thermometers.     Convenient  for  insertion  through  walls  of 
drying  compartment. 

It  has  been  found  that  dry  fruit  may  be  most  readily  removed  from 
the  trays  as  soon  as  it  has  cooled  after  removal  from  the  dehydrater. 
If  an  attempt  is  made  to  scrape  the  fruit  from  the  trays  while  still 
warm  and  soft,  the  fruit  is  broken  and  syrup  is  forced  from  it,  causing 
it  to  stick  to  the  trays;  but,  if  left  several  hours,  it  often  tends  to 
sweat  or  soften  by  diffusion  of  moisture  toward  the  surface  and  is 
then  also  difficult  to  remove.  Often  fruit  which  will  "rattle"  on  the 
trays  when  first  taken  from  the  dehydrater  will  become  softened  and 
pliable  after  several  hours  standing.  This  effect  is  due  to  sweating 
after  case-hardening. 


BULLETIN  337]      SOME  FACTOTS  OF  DEHYDRATER  EFFICIENCY 


295 


Control  of  Humidity. — For  the  rapid  and  uniform  drying  of 
certain  fruits,  especially  halved  pears,  peaches,  and  large  prunes,  it 
is  necessary  to  use  air  of  relatively  high  humidity.  Very  dry  air 
causes  such  fruits  to  case-harden,  a  condition  which  results  in  very 
slow  drying.  Moist  air  permits  diffusion  of  water  outward  to  keep 
pace  with  evaporation  from  the  surface  and  thus  prevents  case- 
hardening.  The  desired  humidity  at  the  air-intake  end  of  the  tunnel 
cannot  always  be  maintained  by  recirculation  of  the  air,  as  the  fol- 


Fig.  7. — A  convenient  and  accurate  form  of  hygrometer  for  the  determination 
of  the  relative  humidity  of  air  in  dehydraters. 

lowing  consideration  demonstrates.  Air  at  the  exhaust  end  of  the 
tunnel,  if  at  110°  F.  and  80  per  cent  relative  humidity,  when  returned 
to  the  furnace  room  and  reheated  to  160°  F.  and  mixed  with  about 
25  per  cent  of  its  volume  of  outside  air,  is  reduced  below  20  per  cent 
relative  humidity,  whereas  it  is  essential  in  some  instances  that  the 
humidity  be  increased  to  35  or  40  per  cent.  It  was  found  that  steam 
admitted  to  the  air  return  duct  did  not  bring  about  the  desired  result 
because  though  the  relatively  cool  air  was  saturated  with  steam  it 
became  greatly  reduced  in  relative  humidity  as  it  passed  through  the 


296  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

air  heating  system.  It  is  necessary,  therefore,  to  introduce  the  steam 
into  the  reheated  air  as  it  leaves  the  air  heating  system  on  its  way  to 
the  drying  chamber. 

Allowing  water  to  drip  upon  the  furnace  and  radiating  pipes  gave 
fair  results.  Much  better  results  were  obtained  by  the  use  of  two 
cyclone  spray  nozzles  set  at  such  an  angle  in  the  furnace  room  as 
to  play  a  fine  spray  of  water  against  the  furnace  and  pipes  and  into  the 
air  stream.  Humidities  of  30  to  45  per  cent  at  temperatures  of  145°  F. 
to  155°  F.  were  easily  maintained  by  this  means  in  the  University 
Farm  dehydrater. 

It  is  recommended  that  all  air  blast  tunnel  dehydraters  be  equipped 
with  an  air  humidifying  device.  Steam  heated  dehydraters  can  easily 
be  equipped  with  open  steam  jets  for  humidity  control. 

Dipping  Eqmpmemt. — Wine  grapes  and  prunes  are  usually  dipped 
in  a  dilute  boiling  lye  solution  before  dehydration  in  order  to  check 
the  skins  and  thereby  increase  the  rate  of  drying.  Several  types  of 
dippers  are  in  use  for  this  purpose.  One  of  these  is  the  common 
hand-power  dipper  found  in  many  prune  dry  yards  in  California. 
This  machine  is  fairly  satisfactory  but  the  heating  equipment  has  often 
proved  inadequate.  "Where  forced  draft  oil  burners  are  installed 
instead  of  wood  burning  grates  or  gravity  distillate  burners,  hand- 
power  dippers  have  been  fairly  satisfactory.  Hand-power  dippers 
may  be  attached  to  a  cam  shaft  and  operated  by  a  small  gasoline 
engine. 

The  revolving  drum  type  of  dipper  used  with  success  for  prunes 
has  not  been  satisfactory  for  grapes  because  of  the  difficulty  of  regula- 
tion and  the  shattering  of  grapes  from  the  bunches. 

A  continuous  draper  scalder  for  grapes  was  manufactured  during 
the  past  season.  It  was  of  large  capacity  and  it  was  found  impossible 
to  furnish  sufficient  heat  by  means  of  a  furnace  and  hot  water  circu- 
lation to  maintain  the  lye  solution  at  the  boiling  point.  However, 
by  connecting  the  dipper  to  a  20  h.p.  boiler,  good  results  were 
obtained.  One  of  the  most  successful  dipping  machines  used  during 
the  last  season  was  a  lye-spray  type  of  peach  peeling  machine.  In  this 
machine  the  fruit  is  carried  on  a  broad,  metal  cloth  conveyor,  beneath 
sprays  of  hot  water  which  heat  the  fruit,  then  through  sprays  of  boiling 
lye  and  finally  through  sprays  of  rinsing  water.  By  regulating  the 
concentration  of  lye  and  the  speed  of  the  conveyor,  dipping  may  be  ac- 
curately controlled.  A  25  h.p.  motor  and  a  25  h.p.  steam  boiler  are  re- 
quired.   A  royalty  must  be  paid  upon  all  fruit  dipped  in  this  machine. 

Regardless  of  the  type  of  machine  used,  it  is  essential  that  the  lye 
solution  be  maintained  at  proper  strength  and  at  or  very  near  the 


BULLETIN  337]      SOME  FACTOTS  OF  DEHYDRATER  EFFICIENCY 


297 


Dry     3ulb 


/  %  / 

/    /  . .  / 

/    /.    ,/  s\ 

RtLATivc  Humidity  Chart 

paromew  2S>92fof  Ha 


Ml 

-3a«  FrjncJsc^,  CjI 

Degrees     fchrefthefj 

Dry         Bulb 


Fig.  8. — Chart  showing  the  humidity  of  air  from  the  wet  and  dry  bulb  ther- 
mometer readings.      (Drawn  by  G.  B.  Kidley.) 

boiling  point.  The  proper  strength  of  lye  solution  can  be  readily 
determined  by  an  experienced  operator  from  the  appearance  of  the 
dipped  fruit,  although  it  is  feasible  to  use  a  simple  method  of  deter- 
mining it  by  means  of  titration  with  a  standard  acid  solution. 

Grapes  should  be  rinsed  in  water  after  dipping,  in  order  to  remove 
adhering  lye  solution,  which  tends  to  darken  the  flesh,  injure  the 
flavor,  and  form  a  white  deposit  on  the  dry  fruit.  Sprays  are  used 
in  continuous  dippers  and  a  second  vat  supplied  with  fresh  water 
should  be  used  with  hand-dipping  outfits. 


298  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

ACKNOWLEDGMENTS 

The  investigations  reported  in  this  bulletin  were  made  possible 
by  funds  from  the  appropriation  for  investigations  in  Deciduous 
Fruits  made  by  the  state  legislature  of  1919. 

The  writers  are  indebted  to  the  many  manufacturers,  owners,  and 
operators  of  dehydraters  who  made  possible  the  securing  of  much  of 
the  data  reported  herein.  They  also  wish  to  express  their  appreciation 
to  Professor  F.  T.  Bioletti  for  helpful  revision  of  the  manuscript. 

SUMMARY   AND   CONCLUSIONS 

1.  The  cost  of  a  dehydrater  erected  by  the  average  fruit  grower 
for  operation  during  a  season  of  only  one  or  two  months  must  be  as 
low  as  is  compatible  with  reasonable  efficiency  if  it  is  to  be  profitable. 

2.  A  completely  equipped  and  satisfactory  dehydrater  can  be 
built  for  $500  or  less  per  green  ton  capacity  per  24  hours. 

3.  The  air-blast  tunnel  type  of  dehydrater  is  the  most  economical 
to  operate  in  regard  to  both  fixed  charges  and  operating  costs. 

4.  For  efficiency,  the  velocity  of  air  across  trays  should  not  fall 
below  500  feet  per  minute,  while  the  total  volume  of  air  per  100  square 
feet  of  tray  surface  should  not  be  less  than  250  cubic  feet  per  minute. 

5.  In  order  to  reduce  static  pressure  and  secure  maximum  fan 
capacity,  all  air  passages  should  be  as  large  in  area,  as  short  in  length, 
and  as  direct  as  possible. 

6.  Inefficiency  will  result  unless  all  the  heated  air  flows  between 
the  trays  of  drying  fruit  and  is  equally  distributed  among  the  several 
trays.  This  can  be  accomplished  readily  by  proper  relative  dimensions 
of  the  drying  chamber  and  trays,  supplemented  by  the  intelligent  use 
of  baffles  and  dampers. 

7.  Multivane  or  steel-plate  fans,  although  more  costly,  more  than 
repay  their  extra  cost  by  their  greater  efficiency,  especially  in  large 
dehydraters,  where  high  static  pressures  must  be  overcome. 

8.  Fruits  which  are  sulfured  should  be  dried  on  wooden  trays, 
preferably  with  slat  bottoms.  Unsulfured  fruits  are  most  rapidly 
dried  on  screen  trays. 

9.  Air  of  20  to  50  per  cent  relative  humidity  is  advantageous  in 
the  dehydration  of  fruits  which  case-harden  readily.  Such  moist  air 
permits  the  steady  evaporation  of  moisture  from  the  fruit  at  relatively 
high  temperatures. 

10.  Prunes  and  grapes  are  most  rapidly  dried  if  previously  dipped 
in  a  boiling  lye  solution.  The  first  requisite  of  any  dipper  is  a  source 
of  heat  sufficient  to  maintain  the  lye  solution  boiling  constantly  during 
operation. 


